1. Field of the Invention
This invention primarily relates to devices for measuring the sense of hearing by free field pure tone testing. It is especially useful for the testing of infants and pre-school children; however it is not necessarily limited to such individuals. Since it is quite difficult to test the hearing of infants and pre-schoolers by the use of conventional earphones, this invention provides a means of determining the hearing threshhold level in a free field by observation of the reflex action of the subject or by the use of electronic monitoring equipment which indicates the subject's awareness of the auditory stimulus. This invention also relates to devices for stimulating biological organisms and animals or measuring the acoustical properties of materials or enclosures. It is useful in the fields of psycho-acoustics and bio-acoustics for the testing and stimulating of animals and biological organisms.
2. Description of the Prior Art
There are several different methods which have been employed in the prior art to test hearing in a free field hearing test system. One typical approach is the use of a warble tone. Allison U.S. Pat. No. 2,768,236 shows a device in which a warble tone is produced by means of frequency modulating methods designed into an audiometer which in turn energizes certain loud speakers. The effect of such a warble tone is to reduce standing waves by only about 20% thus limiting the effectiveness for free field testing. This helpful warble tone method of reduction of standing waves is present only in very expensive clinical audiometers. Goldberg U.S. Pat. No. 3,404,235 utilizes white noise or narrow band noise for the estimation of speech threshhold since such noise does not suffer from the effects of standing waves. Unfortunately, clinical hearing evaluations using such white or narrow band noise yield little useful diagnostic data for hearing evaluation purposes, due to its inherent lack of pure tone quality and resultant distortion of stimulus signal. However, one problem common to all prior art techniques is that standing wave reduction is relatively ineffective and quite expensive. It was also desired that a solution to the problem of further reduction in standing waves be found that was inexpensive and readily adapted to commercially available and commonly used audiometers without further modification. For these and other reasons, the following invention was perfected.